Infectious complications have a major impact on the outcome of marrow transplantation. The identification, prophylaxis and treatment of infection provide data which are used by all the research projects supported by this grant. These data are collected in a database organized and administered by personnel supported in this subproject. The intensive infection control support provided for both the marrow transplant recipients and the research projects of this grant creates a valuable opportunity for studies of infection prophylaxis and treatment. We propose studies which will explore effective means of preventing infection in both laminar air flow and conventional hospital rooms. We will study the practicality of selective bacterial decontamination and its effectiveness as an alternative to total decontamination in reducing infection acquisition and the incidence of GVHD. We will also explore new methods of total decontamination to improve the effectiveness of this procedure. Finally, we propose studies of infection control during the prolonged period of immunoincompetence after engraftment.